1. Field of the Invention
The present invention relates to a digitally controlled power source with an output terminal to which DC-DC converters are connected.
2. Description of the Related Art
In a power circuit having DC-DC converters for one or more channels in a subsequent stage, an instantaneous voltage drop may be caused by starting currents in the DC-DC converters in the subsequent stage. As techniques for reducing such an instantaneous voltage drop, power circuits that suppress start-up of a DC-DC circuit with use of capacitors, zener diodes, and/or the like until an input voltage is made constant and that thereby confine an output voltage value within a specified range are disclosed in Japanese Patent Applications Laid-Open Nos. 2002-049430 and 2005-198454, for instance. A power circuit in which influence of the starting currents and the instantaneous voltage drop are reduced by insertion of a starting current suppressing circuit made of thermistors or the like between a controlled power source and a DC-DC converter is disclosed in Japanese Patent Application Laid-Open No. 10-290524.
FIG. 3 is a schematic configuration diagram of a power circuit for a digitally controlled power source according to a prior art technique.
In the power circuit, an arithmetic processing unit 4 carries out digital computation based on a value of an output voltage Vout1 fed back through an A/D converter 5 and outputs command values based on a result of the computation to a PWM generator unit 3. The PWM generator unit 3 generates and outputs PWM signals based on the command values from the arithmetic processing unit 4 and a switching power circuit 2 outputs a voltage Vout1 based on the outputted PWM signals. The voltage Vout1 outputted from the switching power circuit 2 makes a starting current Iout1 flow through a DC-DC converter 12a and a voltage Vout2 is then outputted. A starting current suppressing circuit 11 is provided in order to restrict an amount of the current flowing instantaneously.
A graph of FIG. 4A illustrates change in the current and the voltages at start-up in the power circuit which is illustrated in FIG. 3 and in which the starting current suppressing circuit 11 made of thermistors or the like is not incorporated and a graph of FIG. 4B illustrates change in the current and the voltages at start-up in the power circuit which is illustrated in FIG. 3 and in which the starting current suppressing circuit 11 is incorporated.
In the power circuit in which the starting current suppressing circuit 11 is not incorporated, as illustrated in FIG. 4A, an instantaneous voltage drop in the output voltage Vout1 of the digitally controlled power source 1 may be caused by an inrush current in the starting current Iout1 at the DC-DC converter 12a in the subsequent stage.
In the power circuit in which the starting current suppressing circuit 11 is incorporated, by contrast, as illustrated in FIG. 4B, the voltage drop in the output voltage Vout1 of the digitally controlled power source 1 can be suppressed by imposition of restriction on the starting current Iout1.
The imposition of the restriction on the starting current with provision of the starting current suppressing circuit, however, causes a problem in that prolongation of rise time for the power source may result in unsatisfaction of a request sequence at the start-up. A method in which voltage fluctuation at the start-up is suppressed by capacitors causes a problem in that necessity of the capacitors having large capacitance inhibits size reduction of the power source.